For vehicle stability control system to function properly under a variety of changing road conditions and driver's inputs, precise estimations of vehicle states are necessarily required. In particular, information on the side-slip angle is critical to vehicle handling and safety control. Since commercial sensors measuring the side-slip angle are not cost effective, estimation methods that use available sensor measurements and vehicle dynamics models are necessarily required. This paper proposes a novel methodology to estimate the side-slip angle which is used as an index of vehicle stability. A side-slip angle observer is designed using the bicycle model and the kinematic model. Here, in order to directly use the lateral accelerometer signal, the lateral acceleration compensation method through the roll angle estimation is proposed. The estimation performance is verified under various road conditions and different driver's inputs using HIL test equipment including commercial vehicle dynamics simulation software CarSim.